The present invention relates in general to controlling switching transients for power switching transistors, and, more specifically, to modified gate drive signals for power inverters of a type used in electrified vehicles wherein switching speed adapts to a total current flow between a DC link and the inverter.
Electrified vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC link capacitor. An inverter is connected between the main buses for the DC link and a traction motor in order to convert the DC power to an AC power that is coupled to the windings of the motor to propel the vehicle. A generator inverter may also be connected to the DC link so that AC power from a generator driven by an internal combustion engine can supply DC power onto the link for recharging the battery and/or powering the traction motor.
The inverter(s) and VVC include transistor switching devices (such as insulated gate bipolar transistors, or IGBTs) connected in a bridge configuration including one or more phase legs. A typical configuration includes a three-phase motor driven by an inverter with three phase legs. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor. The inverter is controlled in response to various sensed conditions including the rotational position of the electric machine and the current flow in each of the phases.
The inverter for the motor may preferably pulse-width modulate the DC link voltage in order to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. Pulse Width Modulation (PWM) control signals are applied to drive the gates of the IGBTs in order to turn them on and off as necessary. In an idealized form, the gate drive control signals are square wave signals that alternate each power switching device (e.g., IGBT) between a fully off and a fully on (saturated) state. During turn off and turn on, it takes time for the device to respond to the change in the gate drive signal. For example, after the gate drive signal transitions from a turn-off state to a turn-on state, conduction through the device output transitions from zero current flow to a maximum current flow within a few microseconds.
The optimal switching speed of a power semiconductor transistor device such as an IGBT is a tradeoff between high stresses which could destroy the device at very fast switching speeds and reduced efficiency and increase power losses at slower switching speeds. In particular, when a switching device is turned off as a result of its gate drive signal (VGE) transitioning to its turn-off voltage level, the voltage across the device (VCE) rises while the device output current (IC) drops. Due to transients, VCE typically overshoots the supply voltage (e.g., the DC link voltage). To prevent these resulting voltage spikes from damaging the switching devices, a typical design uses switching devices with voltage ratings much higher than the supply voltages being used.
In an electrified vehicle propulsion system having more than one separate switching bridge connected to the same DC link (e.g., a VVC and an inverter or two inverters), instances will occur in which two different switching devices will be turning off simultaneously. Since the voltage overshoots can be additive, the voltage spikes can be even higher. Use of switching devices with augmented voltage ratings in order to withstand the spikes results in undesirable increases in parts cost, packaging space, and manufacturing cost.